Here's a short video clip of a basic test run of  the replacement  $28 12v dual drive gearbox coupled up to a pair of Floataboat Paddles. It's just a quick setup, so a squeak or two.

The rc tank gearboxes are not recommended, compared to this  quiet and stable unit they sounded like chaff-cutters ! 

Check out the quick video clip. On 12 volts the paddles look like turbofans in a video even though just 150 rpm..

https://youtu.be/QtFFLHmcrpo

That's it on this Alexander Arbuthnot  build until I clear up a couple of other, so far lengthy,  projects.